Wide Bandwidth Attenuator Input Circuit for a Measurement Probe

ABSTRACT

A wide bandwidth attenuator input circuit for a measurement probe has a Z 0  attenuator circuit coupled in series with a compensated RC attenuator circuit. The series attenuator elements of the Z 0  and the compensated RC attenuator circuits are coupled via a controlled impedance transmission line to the shunt attenuator elements of the Z 0  and the compensated RC attenuator circuits. The shunt element of the Z 0  attenuator element terminates the transmission line in its characteristic impedance. The junction of the series and shunt attenuator elements are coupled to the input of a buffer amplifier. At low and intermediate frequencies, the compensated RC attenuator circuit attenuates an input signal while at high frequencies, the compensated RC attenuator circuit acts as a short and the Z 0  attenuator circuits attenuates the input signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the U.S. Provisional ApplicationNo. 60/575,980, filed Jun. 1, 2004.

BACKGROUND OF THE INVENTION

The present invention related generally to input attenuator circuits forvoltage measurement probes and more particularly to a wide bandwidthattenuator input circuit that combines the attributes of conventionalactive probe input circuits and Z₀ input circuits.

Conventional active voltage probes exhibit finite and reactive inputimpedance characteristics that can load a circuit under test atfrequencies greater than 1 GHz to perturb the measurement being made.The perturbation may be great enough to induce failure in the circuit,or at least, to bring the results of the measurement into question.

Referring to FIG. 1, there is shown a simplified schematic of aconventional active probe input circuit 10 that includes a probe bufferamplifier 12 with a damped compensated attenuator input. The probeamplifier 12 is located in the probe head in order to drive the probecable transmission line that is coupled to the measurement instrument,such as an oscilloscope or the like. The probe amplifier 12 also needsto be located physically close to the probing tip in order to reduceinterconnect parasitics and maintain high frequency response. Acompensated RC passive attenuator having a parallel resistive/capacitivepair R1 and C1 acting as the series elements of the attenuator andparallel resistive/capacitive pair R2 and C2 acting as the shuntelements of the attenuator is commonly used in front of the probeamplifier 12 to increase the probe input dynamic range and reduce theeffective probe input capacitance. The compensated RC attenuatorstructure is used to provide flat transmission response over a broadfrequency range. The simplifies schematic of FIG. 1 also includes aninput damping resistor 14, which is used to adjust the probe risetimeand aberrations. The damping resistor 14 may have some effect on theprobe high frequency loading depending on the probe tip parasitics. Theconventional active probe the impedance is usually very high at lowfrequencies because of the input resistance, but begins to drop off at20 dB/decade due to the effect of the input capacitance.

A newer probe input structure uses a current mode amplifier approach asrepresentatively shown in FIG. 2. The current mode amplifier 20 has aresistive input element 22 coupled to parallel resistive/capacitiveelements R1 and C1. The parallel resistive/capacitive elements R1 and C1are coupled to a coaxial transmission line 24 in the form of a coaxialcable. The other end of the coaxial cable 24 is series coupled to aresistive element 26 that terminates the coaxial cable 24 in itscharacteristic impedance. The resistive element 24 is coupled to theinverting input of a transimpedance probe amplifier 28 that has thenon-inverting input coupled to ground. The inverting input node of thetransimpedance probe amplifier 28 is coupled to the output of theamplifier via a parallel resistive/capacitive elements R2 and C2. Theattenuated input voltage signal is converted to a current signal at theprobe amplifier 28 virtual ground node. The resulting current signal isthen converted by the amplifier feedback components R2 and C2 to abuffered output voltage. Although the passive input network is not aconventional compensated attenuator structure, because of the largecoaxial cable capacitance, the amplifier topology makes the feedbackcomponents R2 and C2 act like the shunt elements of a compensatedattenuator with the probe head components, resistive elements 22, R1 and26, and capacitor C1 acting as the series elements.

SUMMARY OF THE INVENTION

Accordingly, the present invention is a wide bandwidth input attenuationcircuit for an active voltage measurement probe receiving an inputsignal from a device under test. The wide bandwidth input attenuationcircuit has a compensated RC voltage divider network having seriesresistive/capacitive elements coupled to shunt resistive/capacitiveelements. A resistive Z₀ voltage divider network has a series resistiveelement coupled to a shunt resistive element where the series resistiveelement is coupled to the series resistive/capacitive elements of thecompensated RC voltage divider network and the shunt resistive elementis coupled to the shunt resistive/capacitive elements of the compensatedRC voltage divider network. A transmission line couples the seriesresistive/capacitive elements of the compensated RC voltage dividernetwork to the shunt resistive element of the resistive Z₀ voltagedivider network with the shunt resistive/capacitive elements of thecompensated RC voltage divider network coupled in series with the shuntresistive element of the resistive Z₀ voltage divider network. Thejunction of the transmission line with the shunt resistive element ofthe resistive Z₀ voltage divider network is coupled to a bufferamplifier.

The transmission line may be implemented as a co-planar transmissionline or as a coaxial cable that may be a lossless cable or a resistivecable. The shunt resistive element of the resistive Z₀ voltage dividernetwork has a resistive value substantially equal to the characteristicimpedance of the transmission line.

A wide bandwidth differential input attenuation circuit for an activevoltage measurement probe may be formed using two wide bandwidth inputattenuation circuits. Each wide bandwidth input attenuation circuit iscoupled to receive one of the complementary positive and negativesignals of a differential signal. The complementary positive andnegative signals from the input attenuator circuits are taken from thejunction of the first transmission line with the shunt resistive elementof the first resistive Z₀ voltage divider network and the junction ofthe second transmission line with the shunt resistive element of thesecond resistive Z₀ voltage divider network. The differential signal iscoupled to first and second inputs of a differential amplifier.

In a further embodiment, the wide bandwidth differential inputattenuation circuit for an active voltage measurement probe has firstand second high frequency signal paths receiving the differential inputsignal. Each high frequency signal path has a resistive Z₀ voltagedivider network with the resistive Z₀ voltage divider network having aseries resistive element coupled to a shunt resistive element via aseries connected transmission line and a blocking capacitor. Adifferential active low pass filter circuit is coupled across theblocking capacitors with the active low pass filter circuit having a lowpass characteristic matched to the frequency response of the respectivefirst and second high frequency signal paths.

A differential amplifier is coupled to receive the attenuateddifferential input signal with the first input of the differentialamplifier coupled to the junction of the shunt resistive element of thefirst resistive Z₀ voltage divider network in the first high frequencysignal path and the second input of the differential amplifier coupledto the junction of the shunt resistive element of the second resistiveZ₀ voltage divider network in the second high frequency path.

The differential active low pass filter circuit may be implemented as avoltage amplifier circuit having first and second inputs coupled to thefirst and second high frequency signal paths prior to the blockingcapacitors. The voltage amplifier circuit receives the differentialinput signal and generates an amplified low pass filtered differentialoutput signal having complementary positive and negative componentsignals. The positive component signal is coupled to the first highfrequency signal path after the first high frequency signal pathblocking capacitor and the negative component signal is coupled to thesecond high frequency signal path after the second high frequency signalpath blocking capacitor. The differential active low pass filter circuitmay further be implemented as a transconductance amplifier circuit withthe differential input signal being coupled from the first and secondhigh frequency signal paths to the amplifier. As with the voltageamplifier circuit, the transconductance amplifier circuit is coupledacross the blocking capacitors of the first and second high frequencysignal paths with the complementary positive and negative componentsignals of the differential output signal being respectively coupled tothe first and second high frequency signal paths.

The objects, advantages and novel features of the present invention areapparent from the following detailed description when read inconjunction with appended claims and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative schematic representation of a conventionalactive probe input circuit for a measurement probe.

FIG. 2 is a representative schematic representation of a current modeamplifier approach for an active measurement probe.

FIG. 3 is a schematic representation of a first embodiment of the widebandwidth attenuator input circuit according to the present invention.

FIG. 4 is a schematic representation of the first embodiment of the widebandwidth attenuator input circuit used in a differential input circuitaccording to the present invention.

FIG. 5 is further embodiment of the wide bandwidth attenuator inputcircuit according to the present invention.

FIGS. 6A and 6B are schematic representations of alternativedifferential active low pass filter circuits used in the wide bandwidthattenuator input circuit according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 3, there is shown a first embodiment of the widebandwidth attenuator input circuit 30 for a measurement probe of thepresent invention. The wide bandwidth attenuator input circuit has aresistive element R_(D) that is implemented as a series combinations ofa probing tip resistance element R1 and an attenuator resistive elementR2 functioning as a series element of a Z₀ attenuation circuit. Theresistive element is coupled to a first resistive/capacitive pair R_(A)and C_(A) acting as the series elements of a compensated RC attenuatorcircuit. The first resistive/capacitive pair R_(A) and C_(A) is coupledto one side a controlled impedance transmission line 32. The other sideof the controlled impedance transmission line 32 is coupled to atermination resistive element R_(T) for the transmission line 32 thatalso functions as the stunt element of the Z₀ attenuation circuit. Thetermination resistive element R_(T) is coupled to a secondresistive/capacitive pair R_(B) and C_(B) acting as the shunt elementsof the compensated R_(C) attenuator circuit. A buffer amplifier 34having an input capacitance of C_(IN) is coupled to the junction node ofthe transmission line 32 with the termination resistive element R_(T).

The resistance of the probing tip resistive element R1 has a preferredvalue in the range of 50 to 150 ohms. The resistance of the attenuatorresistive element R2 is a function of the characteristic impedance ofthe transmission line and the attenuation factor for the Z₀ attenuationcircuit according to the equation R2=(a−1)×Z₀−Z_(tip) where “a”=theattenuation factor, Z₀ is the resistance R_(T) and Z_(tip) is theresistance of R1. In the preferred embodiment the attenuation factor “a”is 4. The resistive element R1 isolates the parasitics at the probe tipwhile the attenuator resistive element R2 is trimmed to provide thedesired attenuation factor for the Z₀ attenuation circuit. This is incontrast to the conventional active probe input circuit 10 where thedamping resistor 14 is trimmed for risetime and aberrations. Theresistive values of R_(A) and R_(B) are at least an order of magnitudehigher than the resistive values of R_(D) and R_(T).

The controlled impedance transmission line 32 may be implemented as alossless transmission line, such as a coplanar transmission line,microstrip line or the like, or as a resistive coaxial cabletransmission line. The transmission line 32 has a delay T_(D) and aresistance of R_(s). For a lossless transmission line, R_(s) is has avalue of “0” while the resistive transmission line has a value of 10 to50 ohms per foot. In the preferred embodiment, the resistivetransmission line has a length of approximately 6 cm and a resistance ofapproximately 4 ohms. Transmission line theory indicates that for alossless transmission line, a resistive element is sufficient toterminate the transmission line in its characteristic impedance asrepresented by the below equation:${C_{B} + C_{IN}} = \frac{2 \times T_{DS}}{R_{S}}$where T_(DS) is the delay of the transmission line and R_(S) is theresistance of the transmission line. In the case of the wide bandwidthattenuator input circuit 30, the buffer amplifier 34 has a fixed inputcapacitance C_(IN) and C_(B) is in series with the termination resistorR_(T). Where the resistance Rs of the transmission line 32 is “0” C_(B)goes to infinity which indicating a short across the capacitance leavingR_(T) terminating the transmission line. In the case where thetransmission line 32 has resistance, the termination of the transmissionline requires a resistive element in series with a capacitive element.In the case of the wide bandwidth attenuator input circuit 30, Rs has aresistive value of approximately 4 ohms which results in the C_(B)having a capacitive value that is in series with the terminationresistor R_(T).

In operation, DC to low frequency input signals are attenuated by theresistive attenuator R_(A) and R_(B) since the resistive contributionsof R_(D) and R_(T) are insignificant compared to the values of theresistive attenuator R_(A) and R_(B). At intermediate frequencies up toa few Gigahertz, input signals are attenuated by the compensated RCattenuator of R_(A)/C_(A) and R_(B)/C_(B). At frequencies above thebandpass of the compensated RC attenuator, the input signals areattenuated by the Z₀ attenuator circuit of R_(D) and R_(T) where C_(A)and C_(B) are effectively shorts for a lossless transmission line andC_(B) has a capacitive value in series with R_(T) for a resistivetransmission line. R_(T) and the series combination of C_(A) and C_(B)form a frequency domain pole that extends the bandwidth of widebandwidth attenuator input circuit 30 over the conventional active probeinput circuit 10.

It should be noted that the Z₀ attenuator circuit and the compensated RCattenuator circuit may implemented with R2 in series with C_(A) andparallel with R_(A) and R_(T) in series with C_(B) and parallel withR_(B). This implementation is electrically equivalent to having R2 inseries with the parallel combination of R_(A) and C_(A) and R_(T) inseries with the parallel combination of R_(B) and C_(B). At DC and lowfrequencies, the capacitive reactance of the C_(A) and C_(B) act as opencircuits causing the input signals to pass through the resistiveattenuator of R_(A) and R_(B). As the frequencies of the input signalsincrease, the capacitive reactance of C_(A) and C_(B) decreases causingthe input signals to pass through the compensated RC attenuator circuitof R_(A)/C_(A) and R_(B)/C_(B). Above the bandpass of the compensated RCattenuator circuit, C_(A) and C_(B) essentially become short circuitswith the input signals being attenuated by the Z₀ attenuator circuit ofR_(D) and R_(T).

Referring to FIG. 4, there is shown the wide bandwidth attenuator inputcircuit 30 incorporated into a differential wide bandwidth attenuatorinput circuit 40. The differential wide bandwidth attenuator inputcircuit 40 has substantially identical wide bandwidth attenuator inputcircuits 30 coupled to the inverting and non-inverting inputs of thedifferential buffer amplifier 42. The resistive value of R_(DP) is thesame as the resistive value of R_(DN). The resistive value of RAP is thesame as the resistive value of R_(AN) and the resistive value of R_(BP)is the same as the resistive value of R_(BN). C_(AP) has the samecapacitive value as C_(AN) and C_(BP) has the same capacitive value asC_(BN). The resistive values of R_(TP) and R_(TN) are the same. Thedifferential input signal having complementary positive and negativecomponent signals is coupled to the two differential input channels ofthe wide bandwidth attenuator input circuits 30 and attenuated in thesame manner as previously described and applied to the input of thedifferential amplifier 42. The differential amplifier 42 generates anoutput signal that is coupled to the measurement instrument.

Referring to FIG. 5, there is shown a further embodiment of the widebandwidth attenuator input circuit of the present invention. The widebandwidth attenuator input circuit 50 is a differential input circuithaving separate high frequency input signal paths 52 and 54 that arecoupled to inverting and no-inverting input nodes of a differentialamplifier 56. Each high frequency input signal path 52, 54 has a inputresistive element R_(DP), R_(DN) that may consist of resistive elementsR1 and R2 as previously described. The resistive elements R_(DP), R_(DN)are coupled to respective controlled impedance transmission lines 58,60. The controlled impedance transmission lines 58, 60 may beimplemented as lossless or resistive transmission lines as previouslydescribed. The transmission lines 58, 60 are coupled to respectiveresistive termination elements R_(TP) and R_(TN) via blocking capacitorsCAP and C_(AN). The other ends of the resistive termination elementsR_(TP) and _(RTN) are coupled to ground. Each high frequency signal path52, 54 forms a Z₀ attenuator circuit consisting of the input resistanceelement R_(DP), R_(DN), the controlled impedance transmission line 58,60, and the resistive termination element R_(TP) and R_(TN). Each Z₀attenuator circuit in combination with the respective blockingcapacitors C_(AP) and C_(AN) forms a high pass filter circuit. Coupledacross the blocking capacitors C_(AP) and C_(AN) is a differentialactive low pass filter circuit 62 for coupling DC through intermediatefrequency signals to the inputs of the differential amplifier 56. Thedifferential active low pass filter circuit 62 needs to have a low passcharacteristic that is matched to the frequency response of the highpass filter characteristics of the high frequency input paths 52, 54consisting of respectively R_(DP), C_(AP), R_(TP) and R_(DN), C_(AN),R_(TN) to achieve a flat overall frequency response from DC to near thebandwidth of the differential amplifier 56.

FIGS. 6A and 6B illustrate two embodiment of the differential active lowpass filter circuit 62. FIG. 6A illustrates voltage amplifier circuit 70having first and second inputs for receiving the differential inputsignal and generating an amplified, low pass filtered differentialoutput signal coupled to first and second outputs. First and second highfrequency signal paths 52, 54 are tapped prior to the blockingcapacitors CAP and CAN to provide the differential input signal to thefirst and second inputs of a voltage amplifier 72 via input resistorsRAP and RAN where RAP and RAN have resistive values that present a highimpedance to the differential input signal at low frequencies. Withinthe voltage amplifier, one of the complementary positive and negativesignals of the differential input signal is inverted and summed with theother complementary differential signal in a summing circuit 74 toreject common mode signals that may be present on the differential inputsignal. In the specific embodiment of FIG. 6A, the complementarynegative signal is inverted. The summed differential input signal iscoupled to separate amplifier circuits 76, 78 and amplified. The outputof one of the amplifiers 76, 78 is inverted to generate the amplified,low pass filtered differential output signal. Alternately, the separateamplifier circuits 76, 78 could be a single differential output stage.Resistive-capacitive feedback elements R_(BP), C_(BP) and R_(BN), C_(BN)are coupled between the respective first input and output and the secondinput and output. The impedance of the feedback elements varies as afunction of the frequency of the differential input signal producing alow pass filtered differential output signal. The complementary negativelow pass filtered signal is coupled to second high frequency signal path54 after the blocking capacitor C_(AN) via output resistor R_(XP) andthe complementary positive low pass filtered signal is coupled to thefirst high frequency signal path 52 after the blocking capacitor CAP viaoutput resistor R_(XN). The voltage amplifier 72 has low impedanceoutputs resulting in R_(XN) being in parallel with R_(TN) and R_(XP)being in parallel with R_(TP). Therefore, the resistance values for theparallel combinations of R_(XN) with R_(TN) and R_(XP) with R_(TP) needto be set so that the resulting parallel resistances match theimpedances of the controlled impedance transmission lines 58, 60.

The high pass filtered differential signal on the first and second highfrequency signal paths 52, 54 is combined with the low pass filtereddifferential output signal from the voltage amplifier circuit 70 togenerate a common mode rejected differential signal having frequencyresponse from DC to greater than 15 GHz. The common mode rejecteddifferential signal is coupled to the inputs of the differentialamplifier 56. The step response of the common mode rejected differentialsignal at the input to the differential amplifier 56 is the combinationof the step responses of the high pass filtered first and second highfrequency input signal paths 52, 54 and the step response of the lowpass filter voltage amplifier circuit 70 as represented by the timeresponse graph 80. The frequency response of the common mode rejecteddifferential signal at the input to the differential amplifier 56 is thecombination of the frequency responses of the low pass filter voltageamplifier circuit 70 and the high pass filtered first and second highfrequency input signal paths 52, 54 as represented by the frequencyresponse graph 82. A flat frequency response occurs when τ for thecircuit equals C_(BP)×R_(BP)=C_(AP)×(Z₀×R_(DP))/(Z₀+R_(DP)) where Z₀equals R_(TP) and C_(BN)×R_(BN)=C_(AN)×(Z₀×R_(DN))/(Z₀+R_(DN)) where Z₀equals R_(TN).

The circuit of FIG. 6A may also by implemented using other circuitdesigns or methods using off the self components or an applicationspecific integrated circuit (ASIC) so long as the result has a one-pole,low pass response, which in combination with the high frequency paths52, 54 produces a flat low to intermediate frequency response.

FIG. 6B illustrates an transconductance amplifier 90 having atransconductance of “gm” and first and second inputs for receiving thedifferential input voltage signal and generating an amplified, low passfiltered differential output current signal coupled to first and secondoutputs. First and second high frequency signal paths 52, 54 are tappedprior to the blocking capacitors CAP and CAN to provide the differentialinput signal to the first and second inputs of the transconductanceamplifier 90 via input resistors R_(AP) and R_(AN). As with thepreviously described voltage amplifier circuit 70, R_(AP) and R_(AN)have resistive values that present a high impedance to the differentialinput signal at low frequencies. Within the transconductance amplifier,one of the complementary positive and negative signals of thedifferential input signal is inverted and summed with the othercomplementary differential signal in a summing circuit 92 to rejectcommon mode signals that may be present on the differential inputsignal. In the specific embodiment of FIG. 6B, the complementarynegative signal is inverted. The summed differential input signal iscoupled to separate transconductance amplifier circuits 94, 96 andamplified. The output of one of the transconductance amplifiers 76, 78is inverted to generate the amplified, low pass filtered differentialoutput current signal. Alternately, the separate transconductanceamplifier circuits 76, 78 could be a single differentialtransconductance amplifier output stage. The complementary negative lowpass filtered current signal is coupled to second high frequency signalpath 54 after the blocking capacitor C_(AN) via output resistor R_(XP)and the complementary positive low pass filtered current signal iscoupled to the first high frequency signal path 52 after the blockingcapacitor CAP via output resistor R_(XN). The combination of the highimpedance outputs of the transconductance amplifier 90 and the outputresistors R_(XP) and R_(XN) provides a high impedance to R_(TP) andR_(TN). As a result, the resistive values of R_(TP) and R_(TN) are setto match the impedances of the controlled impedance transmission lines58, 60. A flat frequency response occurs when τ for the circuit equalsC_(AP)×(Z₀×R_(DP))/(Z₀+R_(DP)) where Z₀ equals R_(TP) andC_(AN)×(Z₀×R_(DN))/(Z₀+R_(DN)) where Z₀ equals R_(TN).

An advantage of the above described wide bandwidth attenuator inputcircuit 50 is a the reduction in the number of capacitors in the highfrequency input paths 52, 54. In the previously described differentialcircuit, each of the high frequency circuits have two capacitors C_(AP),C_(BP) and C_(AN), C_(BN). Blocking capacitors can generate parasiticsat high frequencies so reducing the number of capacitors in the highfrequency input paths 52, 54 reduces the chances of unwanted parasitics.The ability to terminate R_(TP) and R_(TN) directly to ground improvesthe high speed termination design, allowing termination on an integratedcircuit implementation of the design. In addition, the differentialactive low pass filter circuit 62 does not pass common mode signals atlow frequencies simplifying biasing of the differential amplifier 56since many very high speed differential amplifiers have limited DCcommon mode range. Further, any trimming is done in the differentialactive low pass filter circuit 62 and not in the high frequency inputpaths 52, 54.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

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 10. (canceled) 11.A wide bandwidth differential input attenuation circuit for an activevoltage measurement probe receiving a differential input signal from adevice under test comprising: first and second high frequency signalpaths receiving the differential input signal with each high frequencysignal path having a resistive Z₀ voltage divider network with theresistive Z₀ voltage divider network having a series resistive elementcoupled to a shunt resistive element via a series connected transmissionline and a blocking capacitor, the first and second high frequencysignal paths having a high pass filter frequency response; adifferential active low pass filter circuit coupled across the blockingcapacitors of the first and second high frequency signal paths with thedifferential active low pass filter circuit receiving the differentialinput signal from the first and second high frequency signal paths priorto the blocking capacitors and generating a low pass filtereddifferential signal coupled to the first and second high frequencysignal paths after the blocking capacitors with the differential activelow pass filter circuit having a low pass characteristic matched to thefrequency response of the respective first and second high frequencysignal paths; and a differential amplifier having first and secondinputs with the first input of the differential amplifier coupled to thejunction of the shunt resistive element of the first resistive Z₀voltage divider network in the first high frequency signal path and thesecond input of the differential amplifier coupled to the junction ofthe shunt resistive element of the second resistive Z₀ voltage dividernetwork in the second high frequency path.
 12. The wide bandwidthdifferential input attenuation circuit as recited in claim 11 whereinthe differential active low pass filter circuit comprises a voltageamplifier circuit having first and second inputs coupled to the firstand second high frequency signal paths prior to the blocking capacitorsof the first and second high frequency signal paths and receiving thedifferential input signal and generating an amplified low pass filtereddifferential output signal having complementary positive and negativecomponent signals with positive component signal being coupled to thefirst high frequency signal path after the blocking capacitor of thefirst high frequency signal path and the negative component signal beingcoupled to the second high frequency signal path after the blockingcapacitor of the second high frequency signal path.
 13. The widebandwidth differential input attenuation circuit as recited in claim 11wherein the differential active low pass filter circuit comprises atransconductance amplifier circuit having first and second inputscoupled to the first and second high frequency signal paths prior to theblocking capacitors of the first and second high frequency signal pathsand receiving the differential input signal and generating an amplifiedlow pass filtered differential current signal having complementarypositive and negative component signals with positive component signalbeing coupled to the first high frequency signal path after the blockingcapacitor of the first high frequency signal path and the negativecomponent signal being coupled to the second high frequency signal pathafter the blocking capacitor of the second high frequency signal path.14. The wide bandwidth differential input attenuation circuit as recitedin claim 11 wherein the first and second transmission lines areco-planar transmission lines.
 15. The wide bandwidth differential inputattenuation circuit as recited in claim 11 wherein the first and secondtransmission lines are coaxial cables.
 16. The wide bandwidthdifferential input attenuation circuit as recited in claim 11 whereinthe coaxial cables have a resistive signal conductor.
 17. The widebandwidth differential input attenuation circuit as recited in claim 11wherein the shunt resistive element of the first and second resistive Z₀voltage divider networks have resistive values substantially equal tothe characteristic impedance of the first and second transmission lines.